Continuous emissions monitor of multiple metal species in harsh environments

ABSTRACT

A continuous emissions monitor for the measurement of vapor phase and particulate-based metals in gas streams such as those at coal-fired utility plants, incinerators and manufacturing facilities, in which a pulsed plasma source ( 10 ), utilizing a resonant reentrant microwave cavity ( 12 ) which is powered by a microwave generator ( 34 ), operates at sub atmospheric pressures (&lt;50 Torr.) by using a pump ( 48 ) in order to eliminate quenching of the light emission processes by other species in the gas stream and reduce the background emission and where the pulsed operation of the source reduces background light emission from oxides of nitrogen produced in plasma sources operating with nitrogen and oxygen gases thus enhancing the contrast and signal-to-background; resulting in the instrument having a minimum detection level of 0.01 micrograms/m−3 for mercury, as well as, other metal elements such as arsenic and selenium, and requiring less than 10 Watts of microwave power.

This application claims the benefit of Provisional application Ser. No.60/145,341, filed Jul. 23, 1999.

BACKGROUND OF THE INVENTION

This invention relates to a device to measure mercury and other metalsin flue gas from municipal waste incinerators, electric power utilityplants, and manufacturing plants at concentrations as low as 10 partsper trillion (0.1 micrograms/m⁻³).

DESCRIPTION OF THE RELATED ART

The 1990 Clean Air Act Amendments and other legislation have raised theconcerns over low trace concentrations of metals, especially mercury, influe gas from electrical generation plants, municipal wasteincinerators, and heavy industry. Monitoring will be used to determinethe extent of the environmental impact and offers the possibility ofempirically minimizing it. In particular, mercury has been found insignificant quantities in the lakes and streams of the mid-west of theUnited States. The source of this mercury has been found to be from theair emissions from large stationary combustion systems. Technology tomonitor the levels of mercury may be required by regulations and bytechnology to control emission levels. The mercury levels atincinerators will be in the range of 0.1-10 parts per billion. However,the levels at electric power utility plants are only 0.03-1.0 parts perbillion. For an instrument to measure the lower levels of mercury, forexample, it must have a minimum sensitivity on the order of 10 parts pertrillion. Furthermore, the instrument must make measurements on flue gasthat contains many different molecular gas species that can compromisethe measurement of the trace metals, especially mercury.

One approach to measuring metals in gas streams uses plasma emissionspectroscopy. U.S. Pat. Nos. 5,479,254 and 5,671,045 describe a deviceusing high-powered (300 Watts) and high-pressure microwave plasmaemission spectroscopy to measure metals in gas streams. The plasmasource is continuous in operation. The device uses a shorted-waveguideas the plasma source and it is inserted in the gas stream. A microwavetuner is used to couple the high-powered microwaves to theshorted-waveguide. A high-resolution spectrometer (0.01 nm) is used tomake the measurements. In U.S. Pat. No. 5,671,045 the device is modifiedfor operation in harsh gas and high-temperature environment. There areno provisions for reducing competing emission from flue gas componentsnor enhancing the metals emission. The high-pressure operation requiresthe high microwave power to sustain the plasma discharge. There is nodisclosure of measuring mercury. There are examples of measuringmagnesium, chromium, and iron in a high temperature furnace as well asthe laboratory.

U.S. Pat. No. 5,909,277 represents an improvement to the two patentsdescribed above (U.S. Pat. Nos. 5,479,254 and 5,671,045). Oneimprovement is to swirl the gas flow for improved plasma confinement. Anebulizer was added to provide a controlled amount of an element to thedevice for its calibration. There are no teachings or suggestions toreduce competing emission for flue gas components or enhancing themetals emission.

The same approach is employed for a portable field unit for measuringmetals in gas streams. U.S. Pat. No. 5,825,485 describes the same deviceas in U.S. Pat. No. 5,479,254 described above, but uses a pulsedmicrowave power supply to decrease power consumption so that it canoperate off of batteries which can lighten and shrink the size of theinstrument to make the device portable. Again, there are no explicitprovisions for reducing competing emission from flue gas components norenhancing the metals emission. There are no examples of measurements ormentioning of detection levels.

U.S. Pat. No. 3,843,257 is one of the first uses of a microwave emissiondetector to analyze metals and non-metallic compositions. It operated atpressures of 1 Torr and less where there was an increase of sensitivity.The microwave power was applied continuously and there were no otherprovisions to improve instrument sensitivity or reduce competingbackground emission. There are no examples of measurements or mention ofdetection levels.

To help measure particles in a gas stream U.S. Pat. No. 5,854,431describes a screen to collect particles. After collection the screen isheated to release vapors and particles for analysis in a particle orvapor detector, such as an ion mobility detector. The device is aparticle pre-concentrator utilizing a screen.

U.S. Pat. No. 5,242,143 improves the measurement of trace constitutes ingases with a pre-concentration apparatus. The apparatus uses a sorbentwhere trace gases are sampled at high pressure near atmospheric pressureand desorbs in a carrier gas at low pressure and low flow rates. In thiscase the relative mass of the trace constitutes in the carrier gas ismuch greater than in the sample gas, a clear benefit for a massspectrometer benefit.

The approaches used in the art do not deal with the specific means ofreducing interfering emission from flue gas, or means of reducingquenching of metals emissions of interest. These two effects preventsensitive measurement of metals and specifically mercury atconcentrations as low as 10 parts per trillion in flue gas.

SUMMARY OF THE INVENTION

The present invention measures metals in gas streams by employing plasmaemission spectroscopy. Plasma emission spectroscopy by itself, however,is insufficient for measuring concentrations as low as 10 parts pertrillion in flue gas because of a number of physical issues. Flue gasconsists of an ensemble of gases including nitrogen, oxygen, carbondioxide, sulfur dioxide, nitric oxide, and nitrous oxide. Very highconcentrations of water vapor (18%) are also present.

The plasma source is based upon a resonant-high-intensity reentrantmicrowave cavity (FIGS. 1a and 1 b). The reentrant cavity is cylindricalin shape with a coaxial center conductor connected at one end of thecylinder and on the other end is a small gap. The plasma is formed inthe small gap area. The height of the cylindrical part is adjusted tomatch the resonant condition for a pure coaxial cavity. The size of thegap is adjusted to match the resonant condition for the reentrantcavity. There is a small aperture in the cylinder's wall for themicrowave power to flow into the cavity from the microwave generator.The microwave power flows through a waveguide that is connected to themicrowave cavity. There is a quartz window in the waveguide to provide avacuum seal and allow the transmission of the microwave power from thegenerator to the plasma source (FIG. 1b). Alternately, the window canalso be placed in the cavity in the form of a circular quartz tubemounted through the center conductor and exhaust hole in the cavity(FIG. 1a). In addition, the waveguide can be simply replaced with acoaxial wire feed from the power source. The plasma source can bedesigned to operate in the range of about 30-10,000 MHz. Measurementsare made by pulling a gas stream into the cavity with use of a pump. Thegas enters the cavity through the center conductor through a small holeand flows into the plasma region. The gas then exits the cavity througha small hole on the opposite side wall. The instrument can also be builtwith cylindrical and other microwave cavities.

A first configuration is shown in FIG. 2. A sampling probe is mounted ina flue duct to sample the gas stream. An optional filter may follow theprobe to remove particulate matter from the gas stream. A heated sampleline delivers the gas stream to the plasma source. A flow meter or aflow restriction before the plasma source regulates the gas flow. Theplasma source is pulsed to enhance the trace metal signal compared toplasma emission background. The ultra-violet light emanating from theplasma is coupled to a spectrometer with either a fiber optic cable or alens. The spectrometer resolves the light intensity from the trace metalline (for example, the mercury line near 253.65 nm) and the backgroundlight intensity near the trace metal line. These two light intensitiesare measured with detectors on the output of the spectrometer. The twolight intensities are integrated over many pulses, for example, bybox-car averaging techniques. The trace metal light immediately aftereach plasma pulse is enhanced compared to the background light. Theintegrated intensities after each pulse are subtracted and isproportional to the trace metal density. The detectors can be, forexample, a CCD camera, a photo diode array, or a photomultiplier. Theinstrument is calibrated by measuring a known amount of trace metalvapor in a gas stream.

An absorbent can be utilized to collect the trace metal of interest andsubsequently released for analysis. The absorbent will collect the tracemetal and not much of the gas flow. This use of an absorbent isparticularly effective in the collection of trace metal, mercury e.g.,vapor in gas streams. The trace metal is collected with the absorbent ata pressure below atmosphere (40-300 Torr) to prevent the accumulationand condensation of water vapor near the absorbent. The collection timeis less than 2 minutes and analysis time is less than 30 seconds. Thetrace metal is released by heating the absorbent. The gas flow (0.1-1ml/min) is changed to either argon or nitrogen gas to deliver the tracemetal to the plasma source, and produce a plasma in the source with asmall emission background compared to the trace metal signal by reducingthe levels of oxygen in the plasma source. In this configuration theabsorbent eliminates problems with pressure fluctuations in the plasmasource, reduces background emission and quenching problems, andultimately increases signal-to-noise of the measurement. Furthermore,the absorbent can deal with the problem of large water vapor and acidcontent in the gas streams. For this configuration the plasma source iseither pulsed or operated continuously. Although continuous plasmaoperation is not as detrimental with this second configuration, pulsedoperation still has a significant advantage of reducing the competingemission background. The ultra-violet light emanating from the plasma iscoupled to a spectrometer with either a fiber optic cable or a lens. Thespectrometer resolves the light intensity from the trace metal line andthe background light intensity near the trace metal line. These twolight intensities are measured with detectors on the output of thespectrometer. The two intensities are subtracted and it is proportionalto the trace metal density. The detectors can be a CCD camera, a photodiode array, or a photomultiplier. The instrument is calibrated bymeasuring a known amount of trace metal vapor in a gas stream.

A third configuration enables the measurement of metal content onparticles in gas streams. The first two instrument configurationsmeasure metals in the vapor phase in gas streams. The typical metalsthat have been measured were mercury, arsenic, and selenium because theyare in the vapor phase for temperatures below 140 degrees C. The pulseoperation of the plasma source doesn't provide sufficient average powerto vaporize solid metals or break the bonds of molecular metals. Thesecond configuration is modified (FIG. 4) to measure a larger number ofmetals that are typically in a solid form below 140 degrees C. (e.g.lead, chromium, magnesium, manganese, and zinc). A particle collectionsystem is added to collect particles in gas streams and heat them tohigh temperatures (e.g., >1500 degrees F.) where they melt, theirmolecular bonds are broken, and they enter the vapor state. The particlecollection system consists of a high temperature ceramic in the shape ofa cylinder with a pin-hole in the bottom to allow the heated metals tostream out of the ceramic cylinder. The ceramic cylinder is porous toallow gas to be pulled through the ceramic pores, but trap theparticles. The heating is achieved, for example, with 200 watts of ACpower supply heating Nichrome wire wrapped around the ceramic cylinder.The particle collection system collects particles by pumping a gasstream loaded with particles through the ceramic cylinder. The gasstream flows into the ceramic cylinder, through the porous ceramic, andout a small pumping duct. The ceramic cylinder captures the particlesand passes the gas stream. After collection, a non-porous ceramicshutter closes off the open end of the ceramic cylinder and a flow valveshuts off the gas stream collection. Next a purge gas bottle suppliespurge gas to the ceramic cylinder through its porous walls. Then the ACpower supply heats the Nichrome wire and, through conduction, heats theceramic cylinder to temperatures in excess of 1500 degrees F. When theparticles in the ceramic cylinder reach these temperatures the metalswill melt, boil off, and exit the ceramic cylinder as a vapor and intothe plasma source. The ultra-violet light emanating from the plasma iscoupled to a spectrometer with a fiber optic cable, a lens, or both. Thespectrometer resolves the light intensity from the metal lines and thebackground light intensity near the metal lines. These two lightintensities are measured with detectors on the output of thespectrometer. The two intensities are subtracted and the difference isproportional to the metal density. The detectors can be a CCD camera, aphoto diode array, or a photomultiplier. The instrument configurationcan be used to analyze fly ash in coal fired utility plants,contaminated soil, or particulate from manufacturing plants.

These and other features of the invention will be more fully understoodby reference to the following drawings.

BRIEF DESCRIPTION OF DRAWING

FIG. 1a. is a schematic of plasma source with a quartz tube.

FIG. 1b. is a schematic of plasma source with a quartz window in thewaveguide.

FIG. 2. is an illustration of a configuration for a device to measuremetals in flue gas.

FIG. 3. is an illustration of a configuration for a device to measuremetals in flue gas with an absorber.

FIG. 4. is an illustration of a configuration for a device for measuringthe metal content on particles.

FIG. 5. is a data table showing the linearity and sensitivity of thedevice to measure metals in flue gas.

FIG. 6. shows measurements of metal spectra from fly ash.

FIG. 7. is the emission spectra near the 253 nm mercury line fromnitrogen-oxygen plasmas of different pulse duration.

FIG. 8. illustrates a comparison of small and large gas flow rates inthe operation of the plasma source to generate light from mercury.

FIG. 9. shows the measured spectra near the 253.65 nm mercury line fromthe two different sources of mercury: laboratory calibration source andcoal-fired electric utility plant.

DETAILED DESCRIPTION OF THE INVENTION

During the course of this description like numbers will be used toidentify like elements accordingly to the different figures thatillustrate the invention.

The plasma source 10 is based upon a resonant-high-intensity reentrantmicrowave cavity 12 (FIGS. 1a & 1 b). The reentrant cavity 12 is acylinder in shape with a coaxial center conductor 14 connected at oneend of the cylinder and on the other end is a small gap 16. The plasma18 is formed in the small gap 16 area and makes the reentrant microwavecavity 12 behave like a coaxial cavity. The height 20 of the cavity isadjusted to match the resonant condition for a pure coaxial cavity:

Z=¼λ.

Z is the height of the cavity 20 and λ is the wavelength of the inputmicrowaves. The height of the gap 26 is adjusted to match the resonantcondition for the reentrant cavity 12:$\frac{C\quad a\quad \lambda}{2\quad ɛ\quad d} = {\tan \quad {\left( {b/a} \right).}}$

C is the speed of light, a is the radius 30 of the gap 16, b is theradius 32 of the reentrant.

Matching of these two conditions ensures that the plasma source willalways be in resonance with the microwave generator 34 either with orwithout plasma 18 in the gap 16. There is a small aperture 36 in thecavity's wall for the microwave power to flow into the cavity 12 fromthe microwave generator 34. The microwave power flows through awaveguide 38 that is connected to the microwave cavity 12. There is aquartz window 40 in the waveguide to provide a vacuum seal and allow thetransmission of the microwave power from the generator 34 to the plasmasource 10 (FIG. 1b). Alternately, the window can be replaced in thecavity 12 by a circular quartz tube 42 mounted through a hole 44 in thecenter conductor and a hole 46 in the bottom of the cavity 12 (FIG. 1a).In addition, the waveguide 38 can be simply replaced with a coaxialcable feed from the generator 34. The plasma source 10 can be designedto operate in the range of 30-10,000 MHz. A pump 48 is used to pull agas stream 50 into the cavity 12. The gas stream 50 enters the cavity 12through the quartz tube 42 and enters the plasma 18 region. A secondaperture 52 in the plasma source 10 provides a means of coupling lightout of the plasma 18 region. The gas stream 50 then exits the cavity 12through a small hole 46 in the bottom of the cavity 12. The plasmasource 10 can operate over a pressure range of 1 milli Torr toatmospheric pressure (760 Torr), preferably <50 Torr and >10 milli Torr.The plasma source 10 can also be built with cylindrical and othermicrowave cavities.

A first configuration 54 is shown in FIG. 2. A sampling probe 56 ismounted in a flue duct 58 to sample the gas stream 50. An optionalfilter 60 may follow the sample probe 56 to remove particulate matterfrom the gas stream 50. A sample line 64 delivers the gas stream 50 tothe plasma 18 source. The sample line is preferably heated to atemperature greater than 212° F. preferably at least about 300° F. Aflow meter or a flow restriction 66 before the plasma source 10regulates the gas stream 50 flow. The gas stream 50 is pulled throughthe plasma source 10 with a pump 48. The plasma source 10 is driven witha microwave generator 34. The plasma source 10 is pulsed, for example bypulsing the power supply, to enhance the trace metal signal compared toplasma emission background. The ultra-violet light emanating from theplasma 18 is coupled to a spectrometer 68 with either a fiber opticcable 70, a lens 72, or both. The spectrometer 68 resolves the lightintensity from the trace metal line (e.g., near 253.65 nm for mercury±2nm) and the background light intensity near the trace metal line. Thesetwo light intensities are measured with detectors on the output of thespectrometer 68. The two light intensities are integrated over manypulses with averaging techniques e.g., box car averaging. The tracemetal light immediately after each plasma pulse is enhanced compared tothe background light. The integrated intensities after each pulse aresubtracted and is proportional to the trace metal density. The detectorscan be, for example, a CCD camera 74, a photo diode array 76, or aphotomultiplier 78. The instrument is calibrated by measuring a knownamount of trace metal vapor in a gas stream 50. The instrument 54 cansample gas streams 50 at a rate of about 1 to about 50 liters/min. Theplasma source 10 produces the largest response to the trace metals whenit operates at pressures in the range of 0.5 to 50 Torr.

A second configuration 80 (FIG. 3) utilizes an absorbent 82, such as,activated carbon, to collect the trace metal. A sampling probe 56 ismounted in a flue duct 58 to sample the gas stream 50. An optionalfilter 60 may follow the sample probe 56 to remove particulate matter 62from the gas stream 50. A heated sample line 64 delivers the gas stream50 to the absorbent 82. The absorbent 82 will collect the trace metalbut very little of the other species in the gas stream 50. A high flowrate pump 84 pulls the gas stream 50 from the flue gas stack 58, and aflow-monitor 86 measures the flow rate during the gas sampling. Thetrace metal is collected with the absorbent 82 at a pressure belowatmosphere (40-300 Torr) to prevent the accumulation and condensation ofwater vapor near the absorbent 82. A typical collection time is lessthan about 2 minutes and analysis time is less than about 30 seconds.After trace metal collection on the absorbent 82 is completed, two 3-wayflow valves 88 are switched for the analysis of the trace metalcollected by the absorbent 82. Release of the trace metal can befacilitated by heating the absorbent 82. The gas flow (0.1-1 ml/min) ischanged to either argon or nitrogen gas which can be supplied from apurge-gas bottle 90 to deliver the trace metal to the plasma source 10,and produces a plasma 18 in the plasma source 10. The plasma 18 producedin this manner has a small emission background near the trace metal linein the plasma source 10. The gas flow from the purge gas bottle 90 isregulated with, e.g., an orifice 92 or a needle valve 94. A pump 48pulls the purge gas through the plasma source 10 and maintains apressure of 1-5 Torr in the plasma source 10. The plasma source 10 isdriven with a microwave generator 34. In this configuration 80 theabsorbent 82 eliminates problems with pressure fluctuations in theplasma source 10, removes background emission and quenching problems,and ultimately increases the signal-to-noise ratio of the measurement.Furthermore, the absorbent 82 can deal with the problem of large watervapor and acid content in the gas streams 50. For this configuration theplasma source 10 is either pulsed or operated continuously. Althoughcontinuous plasma operation is not as detrimental with this secondconfiguration 80, pulsed operation still has a significant advantage inreducing the competing emission background. The ultra-violet lightemanating from the plasma 18 is coupled to a spectrometer 68 forexample, with either a fiber optic cable 70, a lens 72, or both. Thespectrometer 68 resolves the light intensity from the trace metal lineand the background light intensity near the trace metal line. These twolight intensities are measured with detectors on the output of thespectrometer 68. The two intensities are subtracted and the differenceis proportional to the trace metal density. Examples of the detectorsinclude a CCD camera 74, a photo diode array 76, and a photomultiplier78. The instrument is calibrated by measuring a known amount of tracemetal vapor in a gas stream. The second configuration 80 of theinstrument can sample gas streams 50 at a rate of 1 to 50 liters/min.The plasma source 10 produces the largest response to the trace metalswhen it operates at pressures in the range of 0.5 to 50 Torr andinstrument flow rates during the desorption of the absorbent 82 in therange of 0.25 to 3000 milliliters/min.

Linearity & Sensitivity Measurements

A continuous emissions monitor using the above second configuration 80for the measurement of total mercury in flue gas was used to calculatelinearity and sensitivity. Laboratory studies using bottled flue gas andmercury from a calibrated delivery system determined the measurementsensitivity and linearity for the second configuration 80 by collectingas containing different amounts of mercury The results are shown in thetable in FIG. 5. The instrument linearity and sensitivity was observedover a range of mercury concentrations of 0.3-14 μg/m³. For collectiontimes longer than 60 seconds the response was no longer linear. Forother configurations using absorbent 82 the linearity has been observedwith concentrations above 100 μg/m. The instrument is sensitive tomercury concentrations as small as 0.1 μg/m³ with a signal-to-noise of3. FIG. 5 also shows that the response of the instrument is not affectedby the presence of water vapor in the flue gas.

Still yet another configuration 98 (FIG. 4) enables the measurement ofmetal content on particles 62 in gas streams. The first two instrumentconfigurations 54, 80 measure metals in the vapor phase in gas streams.The typical metals that have been measured were mercury, arsenic, andselenium because they are in the vapor phase at temperatures below 140degrees C. The pulse operation of the plasma source 10 doesn't providesufficient average power to vaporize solid metals or break the bonds ofmolecular metals. The second configuration 80 is modified (FIG. 4) tomeasure a larger number of metals that are typically in a solid formbelow 140 degrees C. (e.g. lead, chromium, magnesium, manganese, andzinc). A particle collection system 100 is added to collect particles 62in gas streams 50 and heat them to high temperatures (>1500 degrees F.)where they melt, their molecular bonds are broken, and they enter thevapor state. The particle collection system 100 consists of a hightemperature ceramic container 102 with a pin-hole 104 in the bottom toallow the heated metals to stream out of the ceramic container 102. Theceramic container 102 is porous to allow gas to be pulled through theceramic pores, but still trap the particles. The particle collectionsystem 100 is mounted very close to the plasma source 10 to enable theefficient delivery of the hot metal vapor to the plasma source 10. Theheating was achieved with a 200 watts of AC power supply 106 heatingNichrome wire 10 wrapped around the ceramic container 102. Alternatelythe metals can be heated in a gas furnace, or with wave power. Theparticle collection system 100 collects particles 62 by pumping a gasstream 50 loaded with particles 62 through the ceramic container 102.The gas stream 50 flows into the ceramic container 102, through theporous ceramic, and out a small pumping duct 114. The small pumping duct114 is connected to a high flow rate pump 84 to pull the gas stream 50through the ceramic container 102. The ceramic container 102 capturesthe particles and passes the gas stream 50. After collection, anon-porous ceramic shutter 116 closes off the open end of the ceramiccontainer 102 and a thee-way flow valve 88 terminates the gas stream 50collection and a purge gas is delivered to the ceramic container 102. Apurge-gas bottle 90 supplies purge gas to the ceramic container 102through its porous walls. The gas flow from the purge gas bottle isregulated with an orifice 92 or a needle valve 94. A pump 48 pulls thepurge gas through the plasma source 10 and maintains a pressure of 1-5Torr in the plasma source 10. The AC power supply 106 heats the Nichromewire 108 and through conduction heats the ceramic cylinder 102 totemperatures in excess of 1500 degrees F. When the particles in theceramic container 102 reach these temperatures the metals will melt,boil off, and exit the ceramic container 102 as a vapor and into theplasma source 10. The plasma source 10 is connected to a pump 48 to pullthe purge gas into the plasma source 10. The plasma source 10 is drivenwith a microwave generator 34. When the metals enter the plasma 18 itproduces light proportional to each metal's density. The ultra-violetlight emanating from the plasma 18 is coupled to a spectrometer 68 witheither a fiber optic cable 70, a lens 72, or both. The spectrometer 68resolves the light intensity from the metal lines and the backgroundlight intensity near the metal lines. These two light intensities aremeasured with detectors on the output of the spectrometer 68. The twointensities are subtracted and the difference is proportional to themercury density. The detectors can be a CCD camera 74, a photo diodearray 76, or a photomultiplier 78. The particles 62 can be collected atgas flow rates of 1 to 50 l/min. The plasma source 10 produces thelargest response to the trace metals when the metals are delivered withthe purge gas flow rate in the range of 0.25 to 50 milliliters/min. Theinstrument configuration can be used to analyze fly ash in coal firedutility plants, contaminated soil, or particulate 62 from manufacturingplants.

Measurement of Metals on Fly Ash

The configuration 98 was tested in the laboratory for its ability toidentify the metals on fly ash. Fly ash is the particle dust coming outof coal-fired utility plants. One-tenth cm⁻³ volume of fly ash wasloaded into the particle collection system 100. The AC power supply 106was turned on and a ceramic cylinder 102 was heated to temperatures inexcess of 1500 degrees F. A purge gas bottle 90 supplied the purge gasto the ceramic cylinder 102 through its porous walls. When the metalsentered the plasma 18 light proportional to each metal's density wasproduced. The ultra-violet light emanating from the plasma 18 wascoupled to a spectrometer 68 with a fiber optic cable 70. A CCD camera74 was used to measure the emission spectrum resolved by thespectrometer 68. FIG. 6 shows measured spectra. The spectrum shows themeasurement of arsenic, lead, zinc, boron, and cadmium. The thirdconfiguration 98 was also used to measure chromium, iron, magnesium,mercury, and manganese. The third configuration 98 is not limited tothese metals and can be used to analyze fly ash in coal fired utilityplants, contaminated soil, or particulate from manufacturing processes.

Reduction of Background Emission

One aspect of this invention utilizes pulsed plasma for reducing thebackground emission spectrum of plasmas 18. The reduction can beenhanced by pulsing the plasma and simultaneously operating at lowpressure. The pulses are 0.2-10 microseconds in duration with arepetition rate near 1 kHz. The operating pressures are less than 100Torr. The method will work with pulse durations less than 100microseconds and repetition rates less than 100 kHz. When not using themethod of this invention, plasma sources operate at high operatingtemperatures and high pressures that tend to greatly enhance chemicalreactions. This is particularly true when the gas stream includes astrong oxidizer, like oxygen, fluorine, and chlorine. In many cases theproducts of the chemical reactions are responsible for backgroundemission observed in plasma sources. For example, plasma sourcescontinuously operating with oxygen-nitrogen gas streams results in thecreation of oxides of nitrogen. Oxides of nitrogen create a largeemission background near the mercury 253.65 nm line and tends to maskthe mercury emission. A nitrogen gas stream with only 0.5% oxygen in aplasma is sufficient to create a large background near the mercury lineat 253.65 nm. FIG. 7 shows a series of spectra near the mercury line(253 nm) measured by the first configuration 54 with the microwaveplasma source 10 operating in a pulsed mode. For these spectra, themicrowave pulses are 10 microseconds wide at a rate of 1 kHz, and theoperating pressure is 10 Torr. The first spectrum is 1 microsecond afterthe beginning of the microwave pulse. The emission background levels arevery low. The next two spectra which are one and two microseconds laterin time, produce significantly more emission background, and are due tothe production of oxides of nitrogen. Later in time the spectra arelarger. The emission spectrum is extremely large when the plasma source10 operates continuously. Therefore, by limiting the pulse length, theemission background spectrum near the mercury line can be reduced tovery low levels. Operating at low pressure also helps reduce theseemissions because the gas doesn't collide as often with the plasma 18,preventing the gas from heating. Small pulses on the order of 1-5microseconds at a rate of 2-4 kHz introduces only 3-30 watts of averagepower to the plasma 18. Therefore, there is very little energy to heatup the gas and produce oxides of nitrogen.

With the plasma source 10 operating in a pulsed mode, all of the lightcan be collected for a measurement of mercury or any other metal.However, when measuring mercury the mercury signal extends after the endof the plasma pulse, but the background light is extremely small. Theplasma 18 slowly decays after the pulse and is sometime referred to asthe “after glow”. During this time the mercury signal at its,characteristic wavelength of 253.65 nm has its best signal-to-backgrounddue to the emission from the oxides of nitrogen. The detected signalafter every pulse is box-car-averaged for some 100,000 pulses to measuremercury in concentrations on the order of 100 parts per trillion. Thistechnique is applicable to all plasma sources.

Reduction of Quenching

Another aspect of the invention reduces metal emission quenching.Quenching is a process where a molecular specie in the plasma 18 takesthe energy of the electrons that normally excite the specie of interestby means of collisions. Quenching dramatically limits trace metal lineemission in gas streams 50 containing oxygen, nitrous oxide, carbonmonoxide, and carbon dioxide. In a particular example, mercury emissionnear 253 nm is suppressed due to oxygen, carbon monoxide, nitrous oxide,and carbon dioxide. The effect is dramatic and only 1% oxygen atatmospheric pressure is sufficient to suppress the mercury emission.However, by operating at low pressures, below 100 Torr, theconcentrations of the quenching gases are lower and the collisions withthe quenching gases are less frequent. Consequently, the quenchingprocess is reduced. At pressures below 30 Torr, quenching is not much ofa factor and the trace metal emission is greatly enhanced. A preferredpressure for operation of the first 54 and second 98 configurations isnear about 1 Torr. In general, quenching of emission is a well-knownphysical effect, however; the quenching of the mercury emission at the253.65 nm wavelength due to the quenching of oxygen, nitrous oxide,carbon monoxide, or carbon dioxide is something discovered in thedevelopment of the present invention. This low-pressure operationunexpectedly improves the instrument sensitivity by a factor of 10,000.Alternately, the plasma source can be operated with non-quenching gases.Pure nitrogen and argon are not a quenching molecules for mercury at the253.65 nm wavelength. The second configuration uses the absorbent 82that collects mercury in gas streams 50 containing quenching gases, butoperates the plasma source 10 with the non-quenching gases of argon ornitrogen. Non-quenching gases are supplied to the plasma source 10 bythe purge-gas bottle 90 in the second configuration 98.

Metal Emission Enhancement

Another aspect of the invention enhances the metal emission by operatingthe plasma source 10 with an extremely small gas flow (0.1-1 ml/min).This can be achieved in practice in the second configuration 9 (FIG. 3),by using an absorbent 82. Using the absorbent 82 enables sampling offlue gas at high flow rates (2-30 liters/min) and pressures (40-760Torr), and then analysis at low gas flow rates (0.1-1.0 ml/min). Inaddition, the analysis can be conducted the low pressures (1-40 Torr),where it is optimum with the plasma source 10 The metals released fromthe absorbent 82 are entrained in the gas flow going to the plasmasource 10. A small gas flow results in the metals taking a longer periodof time to flow through the plasma 18; the more time a metal atom is inplasma 18, the more emission the metal atom will emit. The amount ofemission determines the measurement sensitivity. Consequently, the smallgas flow during analysis greatly improves the instrument sensitivity.FIG. 8 compares the light intensity of an example 253 nm mercury lineemission generated by a fixed amount of mercury passing through theplasma source at gas flow rates of 5 and 0.5 ml/min. FIG. 8 is plottedas a function of time after the start of absorbent 82 heating to releasethe metals. The smaller gas flow results in a larger light intensitythat continues for a longer period of time.

There are additional benefits in operating the plasma source 10 asdescribed above. The combination of low pressure and pulsed operation ofthe plasma source 10 also has the benefit of lowering the averagemicrowave power required to operate the plasma source 10 to a mere 15watts. This dramatically reduces the cost of the microwave generator 20.The use of an absorbent 82 has the added benefit of isolating the plasmasource 10 from pressure fluctuations and quenching gases that are in gasstreams 50 found in flue gas. In addition, it helps concentrate themetal to improve the signal to noise of the metal measurement.

Operational Experiments at a Coal-Fired Utility Plant

Field measurements were completed at a (300 MW) wet-bottom electricalgenerating boiler burning eastern bituminous low-sulfur coal. The secondconfiguration 80 was used during these field tests (FIG. 3). A probe 56was placed in the duct 58 at the output of the precipitator. Although afilter 60 was shown in the figure to remove particles from the gasstream 50, none was used during these field tests. Past the precipitatorthere was little fly ash. The instrument 80 was mounted adjacent to theduct 58 to keep the heated sample line 64 as short as possible (<4feet). The. gas flow into the plasma source 10 is controlled with astandard vacuum pump 32. The UV measurements were made through a quartzwindow on the plasma source 10. A fiber optic cable 70 transports thelight into the spectrometer 68 with an ultra-violet CCD detector 74. Thefield tests were made over one month time period. Two spectrometers 68were used simultaneously during these experiments: an Acton 300ispectrometer and an Ocean Optics 2000 Series spectrometer. The microwavesource 34 produced 1-microsecond pulses at a rate of 4000 per second.The microwave source 34 frequency is 2.45 GHz. The plasma source 10 wasoperated at a pressure of 2 Torr and at flow rates in the range of0.2-1.0 ml/min.

Experiment 1

The power plant was producing 314 MW of electrical power at the time ofthe measurements. The plant burned a combination of 92% coal and 8%natural gas. The heated sample line was held at a temperature near 100degrees C. The measured mercury concentration was only 0.3 μg/m³. Themeasurements were low because the sample line was not heated to the fluegas temperature in the duct of 140 degrees Centigrade.

Experiment 2

The plant produced 29 MW of electrical power at the time of themeasurements. The plant burned 90% coal and 10% natural gas. Themeasurements were completed with the sample line at the flue gastemperature of 140 degrees C. The mercury concentration was measured tobe values of 0.9 and 1.0 μg/m³. The measurements became reproduciblewith the sample lines heated at 140 degrees Centigrade.

Experiment 3

The power plant was producing 270 MW of electrical power on at the timeof the measurements. The plant burned 100% coal. The heated sample linewas held at a temperature near 140 degrees C. The measured mercuryconcentrations were 0.7 and 0.6 μg/m³.

Experiment 4

The power plant was producing 300 MW of electrical power on at the timeof the measurements. The plant burned 100% coal. The heated sample linewas held at a temperature near 140 degrees C. The measured mercuryconcentrations were 0.9,1.4, and 1.25 μg/m³.

Experiment 5

The power plant was producing 275 MW of electrical power on at the timeof the measurements. The plant burned 100% coal. The heated sample linewas held at a temperature near 140 degrees C. The measured mercuryconcentrations were 1.2 and 1.3 μg/m³.

The conclusion of Experiments 1-5 was that it is essential to maintainthe heated sample lines 64 at temperatures at or greater than the gasstreams 50 being sampled from the flue duct 58. At these temperatures,the mercury concentrations were reproducible and in the range of 0.6-1.4μg/m³. Previous measurements of the mercury concentration at thisfacility using wet chemistry techniques were near 1.5 μg/m³. The mercurylevels measured by the instrument were calibrated with a NIST standardmercury source whose concentration was verified by independentlaboratory analysis. The field measurements were more than a factor of10 above the instrument's 80 minimum detection level. The firstconfiguration 54 was used during the field tests to detect arsenic andselenium in the flue gas.

Experiment 6

To verify that the signals produced by the second configuration 80 inthe field tests at a coal-fired utility plant were only due to mercuryand were not compromised by UV emission from another element near thesame wavelength, a UV CCD camera 74 was mounted on the spectrometer 6.Measurements with the CCD camera 74 were made in the field and in thelaboratory. FIG. 9 shows the measured spectra near the 253.65 nm mercuryline from measurements completed in the field and the laboratory. Thespectra 140 are nearly identical and clearly show a large mercury lineat the appropriate wavelength, verifying the laboratory and fieldmeasurements are indeed due to only mercury and there is no complicationof emission from another element. Two spectrometers 68 were usedsimultaneously during these experiments: an Acton 300i spectrometer andan Ocean Optics 2000 Series spectrometer. The microwave source 34produced 1-microsecond pulses at a rate of 4000 per second. Themicrowave source 34 frequency is 2.45 GHz. The plasma source 10 wasoperated at a pressure of 2 Torr and at flow rates in the range of0.2-1.0 ml/min.

While the invention has been described with reference to the preferredembodiment thereof, it will be appreciated by those of ordinary skill inthe art that modifications can be made to the structure and method ofthe present invention without departing from the spirit and scopethereof.

What is claimed is:
 1. A method for the detection of metals in a gasstream comprising the steps of: a) obtaining a sample of said gasstream; b) passing said sample of said gas stream through a plasmachamber at a predetermined rate of flow and pressure; c) exposing saidsample of said gas stream within said plasma chamber to a plasmadischarge resulting in the emission of a plasma light; d) transmittingsaid plasma light to one or more spectrometers; e) resolving the spectraof said plasma light with said one or more spectrometers; f) detectingthe resolved light at said one or more spectrometers with one or moredetectors; g) analyzing the detected light to determine a measurement ofthe metal content of said gas stream; h) repeating steps a) through g)one or more times; i) averaging the measurements obtained in eachrepetition of step f); wherein the signal-to-noise ratio of said metalcontent measurement, indicated by the comparison of the detected lightemissions of said plasma light corresponding to the metal content ofsaid gas stream sample and the detected light emissions of said plasmalight corresponding to the non-metal content of said gas stream sample,is increased by enhancing the strength of the light emissions of saidplasma light corresponding to the metal content of said gas streamsample, increasing the time that said sample of said gas stream isexposed to said plasma, decreasing the detected light emissions of saidplasma light corresponding to the non-metal content of said gas streamsample, or any combination thereof.
 2. The method of claim 1 wherein thedetected light emissions of said plasma light corresponding to thenon-metal content of said gas stream is decreased by pulsing saidplasma.
 3. The method of claim 2 wherein said plasma is pulsed at afrequency of about 2 to about 4 kHz.
 4. The method of claim 2 whereinsaid plasma pulse duration is about 1 to about 5 microseconds.
 5. Themethod of claim 2 wherein said plasma is a microwave plasma discharge.6. The method of claim 1 wherein the strength of the light emissions ofsaid plasma light corresponding to the metal content of said gas streamsample is enhanced by maintaining the pressure of said gas stream samplebetween about 1 milli Torr and about 760 Torr.
 7. The method of claim 6wherein said gas stream sample is at a pressure between about 10 milliTorr and about 50 Torr.
 8. The method of claim 6 wherein said gas streamsample is at a pressure between about 1 and about 5 Torr.
 9. The methodof claim 1 wherein the time that said sample of said gas stream isexposed to said plasma is increased by maintaining the flow rate of saidsample of said gas stream between about 0.1 to about 50 ml/min.
 10. Themethod of claim 9 wherein said flow rate of said sample of said gasstream is between about 0.1 to about 1.0 ml/min.
 11. The method of claim9 wherein said flow rate of said sample of said gas stream is betweenabout 0.2 to about 1.0 ml/min.
 12. The method of claim 1 wherein saidgas stream sample is obtained by diverting a partial gas stream fromsaid gas stream to said plasma chamber.
 13. The method of claim 1wherein said sample stream is obtained by: passing a partial streamdiverted from said gas stream through an absorbent which removablyabsorbs said metals from said partial stream; stopping the flow of saidpartial gas stream to said absorbent; heating said absorbent tofacilitate the desorption of said metals from said absorbent; directinga flow of an inert gas through said absorbent and thereby removing themetal from said absorbent and incorporating it into said inert gas flow;and, directing said metal containing flow to said plasma chamber;wherein the absorbent is heated directly or by heating said flow ofinert gas.
 14. The method of claim 1 wherein said sample stream isobtained by: passing a partial stream diverted from said gas streamthrough a porous particle collector which removes and collects anyparticles within said partial stream; stopping the flow of said partialgas stream to said porous particle collector; heating said particlecollector to a temperature sufficient to vaporize any metal particlescollected by said particle collector; directing a low pressure flow ofan inert gas through said particle collector, thereby incorporating anymetal vapor into said inert gas flow; and, directing said metalcontaining low pressure flow to said plasma chamber.
 15. An apparatusfor the detection of metals in a gas stream comprising: a plasma chamberhaving an internal cavity, a gas flow input, a gas flow output, anenergy input and a light output; an energy source connected to saidenergy input; one or more spectrometers having an input coupled to saidchamber light output and an output; two or more light intensitydetectors coupled to said spectrometer output for measuring metal andnon-metal content; a pulsing means for said energy source; a gas flowattenuation means for regulating the gas stream entering said gas flowinput; a pump means for maintaining the flow rate and pressure of saidgas flow from said gas flow input, through said cavity and out of saidgas flow output; a means for sampling the gas stream and supplying thesample to said gas flow attenuation means; wherein the signal-to-noiseratio indicated by the comparison of the measurement from the lightintensity detectors corresponding to the metal content of said gasstream sample and the measurement from the light intensity detectorscorresponding to the non-metal content of said gas stream sample, isincreased by pulsing said energy source with said pulsing means,adjusting said gas flow attenuation means and said pump means tomaintain a flow rate below about 50 ml/min, adjusting said gas flowattenuation means and said pump means to maintain a pressure below about760 Torr.
 16. The system of claim 15 wherein said cavity is aresonant-high-intensity reentrant microwave cavity and said energysource is a microwave generator.
 17. The system of claim 15 wherein s aid energy source is a radio frequency wave generator.
 18. The system ofclaim 16 wherein said microwave generator is coupled to said energyinput with a waveguide.
 19. The system of claim 15 wherein said samplingmeans comprises an absorbent means, an inert gas source capable ofsupplying an inert gas at varying rates and pressures, and one or moreflow control means; wherein a portion of the gas stream is diverted toand directed through said absorbent means thereby absorbing metalcontained in said diverted gas stream; wherein said one or more flowcontrol means can be selectively configured individually or in a group,between a configuration which directs the diverted gas stream to saidabsorbent and a configuration which allows said inert gas through saidabsorbent and to said plasma chamber gas flow input.
 20. The system ofclaim 15 wherein said sampling means comprises a particle collectionmeans, a heating means, an inert gas source capable of supplying aninert gas at varying rates and pressures, and one or more flow controlmeans; wherein a portion of the gas stream is diverted to and directedthrough said particle collection means thereby removing and collectingparticles of metal contained in said diverted gas stream; wherein saidheating means heats said collected metal particles to temperaturesgreater than 1500 degrees F. converting said particles into metal vapor;wherein said one or more flow control means can be selectivelyconfigured, individually or in a group, between a configuration whichdirects the diverted gas stream to said absorbent and a configurationwhich allows said inert gas through said absorbent and to said plasmachamber gas flow input.